Death begets failure in the heart (original) (raw)
Cell death can occur by apoptosis, necrosis, or perhaps autophagy. A framework has been described for apoptosis, a highly regulated cell suicide process that is hard wired into all metazoan cells (10). Much less is known about necrosis, although recent work suggests that this form of cell death may be actively regulated and is not necessarily accidental (11, 12). Autophagy is an important recycling process in which macromolecules are degraded in the lysosome so that their components can be used as energy substrates by the cell (13). Whether autophagy functions as a death process remains controversial (11, 14).
Apoptosis is mediated by 2 evolutionarily conserved central death pathways: the extrinsic pathway, which utilizes cell surface death receptors; and the intrinsic pathway, involving mitochondria and the ER (Figure 1) (10). In the extrinsic pathway, death ligands (e.g., FasL) initiate apoptosis by binding their cognate receptors (15). This stimulates the recruitment of the adaptor protein Fas-associated via death domain (FADD), which then recruits procaspase-8 into the death-inducing signaling complex (DISC) (16–18). Procaspase-8 is activated by dimerization in this complex and subsequently cleaves and activates procaspase-3 and other downstream procaspases (19).
In the extrinsic pathway, ligand binding induces death receptors to recruit FADD, which recruits procaspase-8. Within this complex (DISC), procaspase-8 dimerizes and activates. Caspase-8 proteolytically activates procaspase-3, which cleaves cellular proteins causing cell death. In the intrinsic pathway, extracellular and intracellular stimuli signal to the mitochondria through a variety of BH3-only proteins (e.g., Bid) and through Bax, which translocates to and inserts into the outer mitochondrial membrane. Bax and Bak (not shown) stimulate the release of cytochrome c (cyt c) and other apoptogens. Once released, cytochrome c binds Apaf-1 along with dATP. This triggers oligomerization of Apaf-1 and recruitment of procaspase-9. Within this complex (apoptosome), procaspase-9 dimerizes and activates. Caspase-9 proteolytically activates procaspase-3. Caspase-8 also cleaves Bid, and its C-terminal fragment translocates to the mitochondria, where it activates Bax and Bak stimulating apoptogen release. FLIP binds and inhibits procaspase-8 in the DISC. ARC binds Fas, FADD, and procaspase-8, inhibiting DISC assembly. ARC, Ku-70, and Humanin bind Bax and inhibit its conformational activation and translocation. Bcl-2 and Bcl-xL (not shown) inhibit mitochondrial apoptogen release. XIAP binds and inhibits activated caspase-9 and -3. Once cytoplasmic, the mitochondrial apoptogens Smac and HtrA2 bind XIAP, displacing and disinhibiting caspases. HtrA2 also has a serine protease activity that cleaves XIAP (not shown). AIF translocates to the nucleus and, in conjunction with a presumed endonuclease, mediates large-scale DNA fragmentation. A subset of intrinsic pathway death signals stimulate the ER, possibly through BH3-only proteins. This causes the release of intraluminal Ca2+ into the cytoplasm, which is mediated by Bax and Bcl-2 through the IP3R. Ca2+ translocates to the mitochondrial matrix and stimulates opening of the mitochondrial permeability transition pore in the inner membrane (not shown), which indirectly results in apoptogen release. The ER pathway may also activate procaspase-12, which can cleave and activate procaspase-9 independently of apoptosome formation (see text).
In contrast to the extrinsic pathway that mediates a specialized subset of death signals, the intrinsic pathway transduces a wide variety of extracellular and intracellular stimuli including loss of survival/trophic factors, toxins, radiation, hypoxia, oxidative stress, ischemia-reperfusion, and DNA damage. Although a myriad of peripheral pathways connect these signals with the central death machinery, each ultimately feeds into a variety of proapoptotic Bcl-2 proteins that possess only Bcl-2 homology domain 3 (BH3-only proteins) and the proapoptotic multidomain Bcl-2 proteins Bax and Bak (20). These proteins undergo activation through a diversity of mechanisms to trigger the release of mitochondrial apoptogens, such as cytochrome c, into the cytoplasm (21–27). Once in the cytoplasm, cytochrome c binds Apaf-1 along with dATP. This stimulates Apaf-1 to homo-oligomerize and recruit procaspase-9 into the multiprotein complex called the apoptosome (28–39). Within the apoptosome, procaspase-9 is activated by dimerization, after which it cleaves and activates downstream procaspases. Bid, a BH3-only protein, unites the extrinsic and intrinsic pathways: following cleavage by caspase-8, Bid’s C-terminal portion translocates to the mitochondria and triggers apoptogen release (40–42).
The extrinsic and intrinsic pathways are held in check by a variety of endogenous inhibitors of apoptosis. FLICE-like (Fas-associated death domain protein-like-interleukin-1–converting enzyme–like) inhibitory protein (FLIP), whose expression is highly enriched in striated muscle, binds to and inhibits procaspase-8 in the DISC (43). Antiapoptotic Bcl-2 proteins, such as Bcl-2 and Bcl-xL, inhibit mitochondrial apoptogen release through biochemical mechanisms that are still incompletely understood (10). Ku-70 and humanin bind Bax and block its conformational activation and translocation to the mitochondria (44, 45). X-linked inhibitor of apoptosis (XIAP) and related proteins that contain baculovirus inhibitor of apoptosis repeats bind to and inhibit already activated caspases-9, -3, and -7, as well as interfering with procaspase-9 dimerization and activation (46–50). Each of these inhibitors act on circumscribed portions of either the extrinsic or intrinsic pathway. In contrast, apoptosis repressor with a CARD (ARC), which is expressed preferentially in striated muscle and some neurons, antagonizes both central apoptosis pathways (51). The extrinsic pathway is inhibited by ARC’s direct interactions with Fas, FADD, and procaspase-8, which prevent DISC assembly, while the intrinsic pathway is inhibited by direct binding and inhibition of Bax (51–53).
Efficient cell killing usually requires neutralization of inhibitory pathways as well as activation of effector mechanisms. This is best illustrated in mammalian cells by the ability of sympathetic neurons to withstand the usual toxic effects of direct intracellular injection of cytochrome c, a resistance attributable to XIAP (54). In most cell types, XIAP’s tonic inhibition of caspases is relieved by the release of the mitochondrial apoptogens Smac/DIABLO and Omi/HtrA2, which directly bind XIAP, thereby displacing caspases (23–25). In addition, Omi/HtrA2 has a serine protease activity that cleaves XIAP, resulting in its irreversible inhibition (55).
Recently, the endoplasmic reticulum has been recognized as an important organelle in the intrinsic pathway. In addition to its role in cellular responses to traditional ER stresses, such as misfolded proteins, this organelle appears to be critical in mediating cell death elicited by a subset of stimuli originating outside of the ER, such as oxidative stress (56). Similar to their roles in transducing upstream signals to the mitochondria, BH3-only proteins appear to relay upstream death signals to the ER (57). The death signal output from the ER can take several forms. First, certain death stimuli increase cytoplasmic Ca2+ concentrations, a process controlled by Bax and Bcl-2 in the ER membrane through interactions between Bcl-2 and the inositol 1,4,5-trisphosphate receptor (IP3R), an ER Ca2+ release channel (56, 58). Increased cytoplasmic Ca2+ can result in Ca2+ overloading of the mitochondrial matrix and opening of the mitochondrial permeability transition pore. This, in turn, can lead to permeabilization and depolarization of the inner mitochondrial membrane, gross mitochondrial swelling, rupture of the outer mitochondrial membrane, and apoptogen release (59). The relevance of this mechanism may be dependent on cell context and stimulus, however, as apoptogen release in some instances of apoptosis is limited to much more subtle features of mitochondrial remodeling (60). A second ER death output is activation of procaspase-12 (61). Procaspase-12 can be activated through the intrinsic ER machinery (62) or cleavage by calpain, which can, in turn, be activated by elevations in cytoplasmic Ca2+ (63). Once activated, caspase-12 can translocate to the cytoplasm and cleave and activate procaspase-9 independently of apoptosome formation (64). This mechanism may provide a means by which the ER can carry out the distal steps in the intrinsic pathway independently of mitochondria. Although knockout mice have uncovered an essential role for procaspase-12 in ER stress-induced apoptosis (61), the applicability of this mechanism to humans is less clear due to procaspase-12 polymorphisms in some populations that encode nonsense mutations (65).